Materials for engineering, 3rd Edition - (Malestrom)

Composite materials 213
10
9
Type B
8
7
Mixture of
types A and B
σ max /σ L
6
5
Type A
4
3
R = constant
0
10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 9 10 10 10 11
Number of cycles (N)
6.23 Illustration of three types of S–N curves for MMCs.
ultraviolet radiation and, since the bulk of the fibres in the composite are in
any case protected from the radiation by the resin, any degradation from this
cause is likely to be dominated by effects upon the matrix. Carbon fibre may
have an effect similar to carbon black in absorbing ultraviolet radiation and
will alleviate the effect of radiation damage to the polymer by converting it
to thermal energy.
In MMCs, aqueous environments may lead to severe corrosion problems
if the metal/fibre combination forms a galvanic couple. Carbon fibre–
aluminium composites have been identified as being prone to this type
of attack. The principal attack occurs where fibres are exposed, at cut or
machined surfaces for example. Similar problems have been encountered
when carbon fibre-reinforced plastics have been used in conjunction with
metallic components (such as fixing screws) in the presence of an aqueous
environment.
Moisture can cause degradation of properties in fibre-reinforced polymers
if it is transported to or diffuses along the fibre/matrix interface. It may cause
breakdown of bonding between fibre and matrix, thus impairing the efficiency
of stress transfer in the composite. If the fibre/matrix bonding is purely
mechanical in nature, then the properties of the composite would be expected
to recover when it is dried out, but if the bonding is chemical then, unless
these bonds are restored on drying, permanent damage will result.
Glass and Kevlar-49 fibres are themselves affected by quite low levels of
exposure to moisture, being susceptible to stress-corrosion cracking, and the
effect is even more pronounced in acid or alkaline environments. Extensive
stress-rupture tests upon a range of GRP with different resins, in aqueous